For most consumers, the ideal coffee beverage is prepared by brewing roasted and freshly ground coffee beans with hot water; usually at a temperature of 90.degree. C. to 100.degree. C. However, since these conditions produce yields of soluble coffee solids of less than 25%, it is not commercially viable to produce soluble coffee products under these conditions. Therefore most recent technological developments made in the soluble coffee field concern attempts to simulate this ideal coffee beverage while obtaining a viable yield.
Early attempts to raise the yield of soluble coffee solids centered on increasing the temperature and pressure of the extraction liquid used to extract the coffee solids from the roast and ground coffee. However it was soon found that, to reach acceptable yields, the temperatures and pressures needed to be sufficiently high such that extensive hydrolysis occurred in the extraction system. This is undesirable since it results in the production of tars, which cause fouling, and off-flavors.
Attention then turned to subjecting roast and ground coffee to extraction under relatively mild conditions and, separately, subjecting the partially extracted grounds to hydrolysis to increase the yields. In this way, the problem of off-flavors could be reduced to acceptable levels. For example, U.S. Pat. No. 4,158,067 (Wouda) discloses a process in which roast and ground coffee is subjected to counter-current extraction in two stages with a separate hydrolysis stage between the two extraction stages. Hot water at 60.degree. C. to 120.degree. C. is introduced into the second extraction stage while fresh roast and coffee is introduced into the first extraction stage. The extraction liquid leaving the second extraction stage is introduced into the first extraction stage to extract the fresh roast and ground coffee. The partially extracted coffee grounds leaving the first extraction stage are subjected to thermal hydrolysis at a temperature of 140.degree. C. to 200.degree. C. in a separate hydrolysis stage before being introduced into the second extraction stage. In this way, hydrolyzed coffee solids which are extracted in the second extraction stage do not pass through the hydrolysis stage and do not undergo any further extreme conditions. The production off-flavors and tars which otherwise might have resulted, are reduced.
A further development of this concept is disclosed in European patent 0363529 (Kraft General Foods). In the process described in this patent, the partially extracted coffee grounds obtained from a first extraction stage are slurried and then subjected to hydrolysis at a temperature of 200.degree. C. to 260.degree. C. The hydrolysis is carried out for 1 to 15 minutes in order to remove at least 50% of any mannans and to produce a hydrolysate containing less than 50% monosaccharides and less than 10% of polysaccharides which contain more than 6 saccharides in the polysaccharide chain. The hydrolysate is then added to the coffee extracts obtained from extraction stages.
Although this process is described to provide good yields, reasonably large amounts of saccharides of shorter chains and lower molecular weights are produced. This significantly alters the taste and mouthfeel of beverages produced from the resulting soluble coffee products. Certainly, the flavor profile no longer matches that of brewed coffee. Also, the increased amount of saccharides of smaller chains causes the extract obtained from this process to behave differently, when compared to standard extracts, during concentration and drying. In particular, the extract has a lower freezing point which makes freeze-drying more difficult while stickiness becomes a problem during, spray-drying. Further, the stability during storage of coffee powders produced from the extract is reduced due to enhanced hygroscopicity and lower glass transition temperatures. The extent of the problem is of course dependant on the severity of the hydrolysis treatment.
Therefore there is still a need for a process for extracting soluble coffee solids which has good yields and which provides a soluble coffee product having saccharides of higher molecular weight. There is also a need for a soluble coffee product which contains hydrolyzed coffee solids and saccharides of higher molecular weight.